Things I regret not buying more of (I'm not in the UK so I don't know if you can get these easily)
Maybelline Snapscara Mascara (my fave is the cherry one)
Elf eye shadow
Gas relief tablets! I am an ibs sufferer and pay a fortune for Iberogast drops. In the US pharmacies are racks of stomach related items. I did have a laugh at the name, got a huge tub of 100 for next to nothing. They really worked well, and now I am back to paying plenty for the drops instead.
Not sure if you get them in the UK, but my dd is a dancer, and in US you can buy the bobby pins that are slightly bent to fit the shape of the head better when putting hair into a bun for eg. Hardly the most crucial thing in the world, but I thought pretty inventive and they worked so well. Of course we didn't buy enough of those, as we all know how they go missing.
Pink range underwear from Victoria Secret. Their bras just fit me so well and I cannot find an exact replacement.
Sports wear. We found Old Navy leggings amazing quality for the price. None of this bend over and see your knickers nonsense with them. They have been so well worn, and you would never say, they wash so well and keep their shape.
I am sure there will be more if I mull over it long enough...